Carbon is a very versatile element; its compounds are the basis of life on Earth, hydrocarbons are by far the most common source of fuel, and the properties of its allotropes vary widely. Carbon allotropes include graphite, diamond, amorphous carbon, C60 and carbon nanotubes.
Carbon nanotubes (CNTs) have received a lot of attention since their discovery due to their electrical, mechanical, optical and thermal properties. Carbon nanotubes can be utilized in different ways, as filler materials, such as in nanocomposites and fibers, or in freestanding form, macroscopic CNT wafers (also known as buckypapers or CNT mats) and CNT fibers. Carbon nanotubes tend to aggregate due to the van der Waals force between them. This limits their applicability as functional fillers in other materials or as free standing CNT films. Improving the dispersion of CNTs in composites has always been a challenge. Due to aggregation, they typically form bundles, and the bundles form globules, which is typically detrimental for the intended functional properties of the composite, such as electrical conductivity, mechanical, magnetic and optical properties. For electrical conductivity, aggregation increases the percolation threshold and may even prevent conductivity at higher CNT content.
Various solvents, surfactants, and processing techniques have been studied to improve CNT dispersion in polymers. While some solvents have been shown to be more effective in dispersing nanotubes, such as nitromethane and DMF, others, such as toluene and methyl ethyl ketone are not as good. Liu, J., T. Liu, and S. Kumar, Effect of solvent solubility parameter on SWNT dispersion in PMMA. Polymer, 2005. 46(10): p. 3419-3424. The solvent-CNT interaction plays an important role in the dispersion of the CNTs, with a better interaction leading to less aggregation of the nanotubes. Dispersing CNTs can be done in several ways. Depending on the dispersing media, solvent or polymer, different approaches can be used. In solvents, sonication, homogenization and microfluidization have been shown to be effective to disperse nanotubes. Luo, S., T. Liu, and B. Wang, Comparison of ultrasonication and micro fluidization for high throughput and large-scale processing of SWCNT dispersions. Carbon, 2010. 48(10): p. 2991394; Badaire, S., P. Poulin, M. Maugey, and C. Zakri, In Situ Measurements of Nanotube Dimensions in Suspensions by Depolarized Dynamic Light Scattering. Langmuir, 2004. 20(24): p. 10367-10370. These methods shear, cut and debundle nanotubes, resulting in smaller bundles and mostly shorter nanotubes. However, after debundling, the nanotubes are free to bundle again and form aggregates. To prevent re-aggregation, surfactants are added to the suspension. These surfactants have hydrophilic and hydrophobic ends and facilitate suspending CNTs in less favorable solvents, such as water. The hydrophobic end interacts with the CNTs while the hydrophilic tail interacts with water for dissolution. Rastogi et al. investigated four different types of surfactants, Triton X-100, sodium dodecylsulfate (SDS), Tween 20, and Tween 80, with CNTs, concluding that Triton X-100 is the most effective between them, resulting in bundle size as small as 4 nm, without specifying the number of walls and outer diameter of the carbon nanotubes. Rastogi, R., R. Kaushal, S. K. Tripathi, A. L. Sharma, I. Kaur, and L. M. Bharadwaj, Comparative study of carbon nanotube dispersion using surfactants. Journal of Colloid and Interface Science, 2008. 328(2): p. 421-428.
Polymers have also been used to improve CNT dispersions. Star et al. used the rigid polymer poly(metaphenylenevinylene) (PmPV) to improve SWNT dispersion in DMF. Star, A., J. F. Stoddart, D. Steuerman, M. Diehl, A. Boukai, E. W. Wong, X. Yang, S.-W. Chung, H. Choi, and J. R. Heath, Preparation and Properties of Polymer-Wrapped Single-Walled Carbon Nanotubes. Angewandte Chemie International Edition, 2001. 40(9): p. 1721-1725. The polymers that have been shown to wrap on carbon nanotubes include 9,9-dioctylfluorene derivatives, polyvinyl pyrrolidone, polystyrene sulfonate, DNA, polybenzimidazole (PBI), aromatic polyimide, poly[(m-phenylenevinylene)-alt-(p-phenylenevinylene)] (PmPV). It was found that as the polymer concentration was increased, the dispersion improved, from an average bundle diameter of 7.1 nm to 3.2 nm using atomic force microscopy (AFM), and the bundle diameter distribution narrowed down, indicating that the polymer had wrapped bundles, not individual CNTs.
Dispersing CNTs in many polymers including poly (methyl methacrylate) (PMMA) has been extensively studied. Considering CNT/PMMA nanocomposite, some studies have suggested different methods to improve CNT dispersion in the matrix. One study reported improvement of the dispersion by melt mixing solvent-casted multi wall carbon nanotube (MWNT)/PMMA films, after several steps of melting and drying as evidenced by optical micrographs; even after 20 melting cycles still some particles existed. Haggenmueller, R., H. H. Gommans, A. G. Rinzler, J. E. Fischer, and K. I. Winey, Aligned single-wall carbon nanotubes in composites by melt processing methods. Chemical Physics Letters, 2000. 330(3-4): p. 219-225. Other studies have suggested functionalization of carbon nanotubes with nitric, sulfuric acid or hydrofluoric acid, or introducing functional groups such as carboxyl, hydroxyl or carbonyl on the sidewall of CNTs which in turn improve the interaction between CNTs and different polymers.
Another approach is in-situ polymerization of polymers with CNTs. In this process the monomers and CNTs are added to the solvent along with initiators. The initiator can open n-bonds of the CNTs, hence chemically connecting the CNTs with polymer chains. These studies generally rely on TEM, SEM, rheological and optical measurements to conclude better dispersion. The in-situ polymerization technique is particularly effective for the preparation of thermally unstable and insoluble polymers, which cannot be processed by melt or solution processing. While in-situ polymerization of polymers with SWNTs leads to a better dispersion, this does not mean the nanotubes have been completely individualized. Though generally the tensile properties improve with this method due to improved dispersion and adhesion, reduction in tensile properties has also been reported.
Helical wrapping of polymers has been suggested as early as 1998. Tang, B. Z. and H. Xu, Preparation, Alignment, and Optical Properties of Soluble Poly(phenylacetylene)-Wrapped Carbon Nanotubes. Macromolecules, 1999. 32(8): p. 2569-2576. While several experimental studies report polymer wrapping on SWNT and/or MWNT, they do not provide evidence of ordered polymer wrapping. The only evidence which pertains to ordered polymer wrapping on carbon nanotubes, to date, comes from computational studies.
Evidence of ordered helical PMMA wrapping on CNTs has never been reported. The only account of ordered helical wrapping is of wrapping of syndiotactic PMMA (st-PMMA) around C60. X-ray diffraction showed an ordered structure of st-PMMA around the buckyballs. The helical pitch of PMMA was ˜0.9 nm, with the possibility to incorporate larger buckyballs, C70 and C84 with slight alteration of the helical pitch. Syndiotactic PMMA has also been shown to crystallize using solvent-induced crystallization with a helical pitch of 0.885 nm. But there has been no report of atactic PMMA crystallization or helical wrapping.